Containerd series - what is containerd?

This article will be about containerd To introduce
Reference resources ：
Introduction to containerd - Phil Estes, IBM $ Derek McGowan, Docker

1. What is? containerd？

• containerd It's a “container runtime”： What is? container runtime？runtime It refers to the container runtime , in other words containerd Given container runtime support . But it should be noted that ：containerd It is not a real runtime , It has the ability of runtime to some extent （ Mainly due to its downstream runtime Support ）. As can be seen from the above figure ,containerd It undertakes the functions of higher-level container management （ for example Docker,K8S）, The lower layer is equipped with a lower level runtime（ for example runc wait —— About runc You can refer to Series articles ）.
• containerd Is a resource manager （resource manager）：containerd You can manage the lifecycle of containers , Involving the creation of containers 、 Delete and so on ; Images can be managed ; You can manage file system snapshots ; You can manage the meta information and dependent information of the container .
• containerd It is a tightly coupled project （tightly scoped）：containerd Ben's body size is very much , Many functions are built on the extension system .

2. containerd The history of

• Original containerd It's just Docker A companion application running , As the connection docker And the ground floor runc Middleware
• With continuous development ,containerd Gradually from a container supervisor（ That is, it only has basic container detection and execution functions ） Degenerate into a fully functional container runtime（ The whole process ）
• although containerd The birth and docker Interwovenness , however containerd Designed a new container and image management interface
• CRI As a plug-in independent of container process, it has been integrated into containerd in , send containerd Be in line with CRI Standard container runtime

3. Why use containerd？

• K8S Users ：K8S At present, it is the most widely used container management project in industrial applications , and K8S Yes containerd With considerable and good support ,containerd stay K8S Is also the most mature .
• For developers ： If you are a developer , Use docker kit Do image development , Then you have used it in real time containerd,docker Itself is also right containerd Supported
• Edge development ： If you are a developer of edge applications ,containerd Due to the small volume , The advantages of high efficiency are widely used in edge virtualization scenes , for example K3S（ It can be understood as K8S A smaller version of , Applicable to edge scenes , Please take a look at the exam here ）
• For service providers ：containerd It can well support external service access , Reference resources faasd,IBM Cloud Function etc.
  
• Program stability ：containerd Of daemon Even if it collapses, it will not affect the running containers and programs
• Efficient use of resources ：containerd daemon The memory space and resources required are very small （ So it also applies to edge scenes ）
• File system resource management ：contianerd Provides a garbage collection mechanism , For redundant and unused file system resources ,containerd It can be recycled automatically , Reduce the invalid occupation of file system resources

4. containerd Architecture （containerd1.4.x）

4.1 The overall structure

• overall contianerd from Client、API、Core、Backend（ Or you could say API、Core、Backend It's a whole , by containerd core）、Shim It consists of five parts , Build on the system and image warehouse
• Control information and data from Client Flow right , Know the container runtime shim

4.2 Client

• Client It's user use containerd Direct interaction objects in the process , about containerd For beginners ,Client It is the module that should be introduced first , It provides rich functions and flexibility for exploring new features and extensions
• Container Management The sub modules are Client Module with container runtimes Sub modules for interaction ： This sub module can provide OCI specification establish ,snapshot Creation and other functions
• Image Distribution yes Client The sub module of image management in , Provides image import , export , Pull , Push and other functions
• containerd The project tries to keep core Part of the concise refinement , therefore Client Part also undertakes more functions and requirements accordingly
• Client The module can be self configured —— It means Client Very flexible

4.3 containerd core（API、Core、Backend）

• This section defines containerd The types of data structures used in and API Interface
• Why? containerd To provide so many interfaces （ whether containerd Too dependent on the use of interfaces ）？—— In fact, these interfaces and data types are set for containerd core Become an intermediate station for data information forwarding , Based on the definition of data structure type ,core It can track the data passing through this part for better use
• Data from core Part of it flows through and is core Track and record , That means core part （ And some of them plugins） No need to store data —— It's also convenient core Medium GC Modules work , Reduce the risk of challenges such as data inconsistency
• Yes Plugin The support of containerd core An important feature of , This makes containerd It has good scalability and flexibility
• plugin Can be like core Send a message ,core You can also ask plugin Send a message ,backend It is also pluggable （plugable）—— for example core Medium Snapshots That is to say plugin The way to achieve
• Each plug-in can be self configured , These configurations will also be included containerd In the main configuration of
• If we have a service application , So it can also be done through grpc Way and core Interact —— for example core Medium CRI The module itself exists as a service , adopt grpc Way and core Interact
• By default Client Will pass proxy Service opportunities grpc And core Interact

4.4 Shim

• containerd core Medium Runtime The sub module can start a new runtime shim, It can pass oci spec And other control information to Shim
• Shims Is really owned container processes The role of control ,Shim As container The direct parent process of exists （ I don't know whether it's right here ）
• containerd core Through a lightweight grpc protocal ttrpc And Shim Interact ; If you restart containerd, that containerd Need to reconnect with shim Establish a connection to pass instructions and data
• There are already many different Shim Realization , One of the most widely used is runc
• runhcs Is running on the windows Upper Shim